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by ReleaseCandidat 715 days ago
> Despite what another comment says, unification is a valid - and rather convenient - way to implement pattern matching

The article shows that it is a valid and rather convenient way. That's why it is important to tell people that it is not necessary to use it for the "usual" pattern matching. You do (again: for the "usual" pattern matching without "variables", as in free variables, on the right side) _not_ want to use unification but "compile" the pattern matching.
1 comments
contificate 715 days ago
If we are talking about the context of a compiler, I agree: you should compile pattern matching to decision trees (DAGs), by way of something like Pettersson's algorithm. In my comment, I specified "in an interpreter", as I think it's the natural way to implement an SML-like match construct in that context.

EDIT: Perhaps I am missing Elixir-specific context and I apologise if this is the case. It was the unification that made me click this post.

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ReleaseCandidat 715 days ago
Well, I'd do that with "real" interpreters (using bytecode) too. But on the other hand also _especially_ for "toy" tree walking interpreters, to learn how to build the pattern matching tree -- as unification is needed for the type checker anyways ;). Although I would start by reading Wadler's version (that I linked in the other post), it's IMHO a bit more accessible than Peterson's https://www.classes.cs.uchicago.edu/archive/2011/spring/2262...

But please don't get me wrong: I'm just saying that there exist other, specialised and (well, generally) more performant algorithms for pattern matching and people should have at least heard about them. If somebody decides to use unification for pattern matching, that still could be changed if performance or better error messages or ... matters.

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contificate 715 days ago
Ah, I thought your username was familiar: you recommended Wadler's approach on a previous HN thread concerning my blog post about Pettersson/Maranget's algorithm (https://news.ycombinator.com/item?id=39241776). Yeah, I admit that Pettersson's telling of the algorithm is a bit turgid (probably why Maranget's paper - which is the same technique as its core, at least to my understanding - is cited more often).
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